compared to the non-relativistic result p = mv = x10^kg m/s
which would then be in error by %.

In the above calculations, one of the ways of expressing mass and momentum is in terms of electron volts. It is typical in high energy physics, where relativistic quantities are encountered, to make use of the Einstein relationship to relate mass and momentum to energy. In relativistic mechanics, the quantity pc is often used in momentum discussions. It has the units of energy.

For extreme relativistic velocities where

then

A useful application of the quantity pc is in the calculation of the velocity as a fraction of c.